BEIRUT (Reuters) - Dozens of air strikes and shelling hit parts of the last swathe of Syrian territory still held by rebels on Friday, according to rebels, rescue workers and a war monitoring group, in a possible prelude to a full-scale government offensive.

The UK-based Syrian Observatory for Human Rights said dozens of strikes from helicopters and war planes had hit parts of Hama, Idlib and Aleppo provinces, killing at least 29 people.

The three areas in northwest Syria are the last major ones still in the hands of fighters seeking to overthrow President Bashar al-Assad, whose forces recaptured the area around Damascus and the southwest earlier this year.

Abu al-Baraa al-Hamawi, a rebel leader in northern Hama, said a "massacre" had resulted from strikes in western Aleppo on the town of Urem al-Kubra. Pro-opposition television Orient News said at least 20 people were killed there and the Observatory said at least 18 had died.

On Thursday the Syrian army had dropped leaflets over Idlib province, urging people to agree to a return of state rule and telling them the seven-year war was nearing its end.

Al-Hamawi said Friday's attacks - the heaviest in months - appeared part of a "psychological media war against the people".

Rebels were "ready to repel any possible attack by regime forces. They will be taught a lesson they won't forget: Idlib is different to other regions," he added.

Mohammed Rasheed, a spokesman for the Jaish al-Nasr rebel group based in Hama province, said pro-Syrian government forces had not yet advanced on the ground and the attacks consisted of shelling and air bombardment.

The Syrian White Helmets, a group of rescue workers established in rebel-held areas of the country, said on Twitter that barrel bombs had also been used.

Syrian state news agency SANA reported that the Syrian army carried out operations against "terrorist groups", its term for the rebels, in northern Hama countryside, destroying several of their headquarters and killing and wounding an unspecified number.

NEXT TARGET?

Hundreds of thousands of Assad opponents have relocated to northwest Syria under evacuation agreements reached as other parts of the country fell to pro-government forces backed by Russia and Iran.

Assad has indicated the area, which borders Turkey, could be his next target

The Observatory said Friday's air strikes were the first to hit the area in almost a month and could be seen as "preparation for an offensive". It has said additional government forces have been arriving this week for a possible attack in an area to the southwest of the opposition territory.

The United Nations worries that such an offensive could force 2.5 million people toward the Turkish border.